Calm girl without a doubt (7)
I believe the answer is:
mollify
'calm' is the definition.
(I know that mollify is a more specific form of calm)
'girl without a doubt' is the wordplay.
'girl' becomes 'molly' (girl's name).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'a doubt' becomes 'if' (as in 'it's a big if').
'molly' enclosing 'if' is 'MOLLIFY'.
(Other definitions for mollify that I've seen before include "Soothe feelings of" , "Placate, appease" , "Appease the anger of (someone)" .)